1. Field of the Invention.
This invention relates to wireless transmitters and receivers and, more particularly, to frequency mixers.
2. Related Art.
A key principle of a frequency mixer is that, in mixing multiple voltage signals together, it adds and subtracts their frequencies to produce new frequencies. In the field of signal processing, the process of multiplication in the time domain is recognized as equivalent to the process of convolution in the frequency domain. Mixers produce distortion or multiplication products that reduce or diminish the quality of the output signal. Much of the art and science of making good use of multiplication in mixing goes into minimizing these unwanted multiplication products (or their effects) and making multipliers provide their frequency translations as efficiently as possible.
Mixers also create nonlinear distortion. Nonlinear distortion may take the form of harmonic distortion, in which integer multiples of input frequencies occur, or intermodulation distortion (IMD), in which different components multiply to form new components. Any departure from absolute linearity results in some form of nonlinear distortion.
Standard mixer design involves significantly nonlinear multiplication. Typically, the switching operation of the mixer causes the local oscillator signal (xe2x80x9cLOxe2x80x9d) to act effectively as a square wave. There are several advantages to such switching action, including reduced noise, improved gain, insensitivity to device mismatch and variation, insensitivity to exact LO strength, and simplified design. A disadvantage, however, is that odd-order mixing products (xe2x80x9cOMPsxe2x80x9d) are generated. An OMP is defined as the product of one input and an odd harmonic of another input.
In many situations only one of the frequency components, such as, for example, finputxe2x88x92flo or floxe2x88x92finput, is of interest and all other products are removed through filtering or image rejection. This approach works well if all of the frequency products to be suppressed by filtering are sufficiently far from the desired frequency. Problems can arise, however, if there is an unwanted signal present on the input that has a frequency that is approximately equal to the frequency of the input signal plus twice the LO frequency (funwanted≈2*flo+finput) An unwanted signal with such a frequency can cause interference between the output signal and a third-order mixing term corresponding to the unwanted signal because the frequency of the third-order mixing term may be very close to the frequency of the output signal (funwantedxe2x88x923*flo≈finputxe2x88x92flo)
In certain applications, an OMP can drastically impair performance. One application where OMPs are especially problematic is in modulators inserted within phase-locked-loops such as, for example, translational loops typically found in Global System for Mobile Communications (GSM) transmitters. In such applications, the use of a phase detector results in modulated harmonics falling close to the intended signal and generating out-of band spurious emissions. A second application where OMPs can cause problems is within a near-zero intermediate frequency (IF) receiver. In such receivers, odd harmonics corresponding to an LO can result in signal interference. What is needed then is a system for preventing odd-order mixing products from significantly degrading the quality of a frequency mixer output.
In one embodiment of the invention, a low harmonics mixing system produces a mixer output having significantly fewer odd-order mixing products than a standard mixer. More specifically, third-order mixing products caused by the presence of LO third harmonics can be substantially eliminated by dividing an LO signal into two separate signals having a predetermined phase difference, mixing each of the two signals with an input signal to produce a mixed signal, and then combining the mixed signals.
In another embodiment of the invention, an LO signal is first divided into two signals having a first predetermined phase difference. Each of the two signals is further divided into two mixing signals having a second predetermined phase difference and each of the mixing signals is mixed with an input signal to produce a mixed signal. Then, the mixed signals are combined to produce a mixer output having substantially no third-order, fifth-order, or ninth-order mixing products.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.